Closures of plastics materials for receptacles for fluent substances have been known wherein a membrane closes the pouring opening but which is adapted to be punctured at the time access to the contents is desired. However, those known to me are unnecessarily complicated, unreliable, and include features mitigating against economical molding. Moreover, parts of the device have been formed so as to protrude beyond the generally circular outline such devices are given in order that the same may be assembled rapidly by automatic machinery, viz. by the use of a spinning chuck which grips the device and rotates the same into tight engagement with the neck of the receptacle by means of interengaging threads. For these and other reasons the packaging industry has not adopted them.